Chez Vince - S4-E41
Continuity mistake: In the cafeteria, TJ is wearing a suit to welcome the customers. But when he and the rest of the guys go to the teacher's lounge he's back in his normal clothes.
Continuity mistake: Gretchen starts talking to that man who doesn't talk, but later when Mikey is singing for the first time, you see that man booing him, then a minute later Gretchen is still talking to him.
Lawson and His Crew (2) - S5-E12
Continuity mistake: After Vince discovers that Mrs. Finster's new cactus is missing from the windowsill, we see that Swinger Girl has it with her, on top of the roof of the school. However, when the shot changes to the new group (With Lawson, Skeenz, Kurts, Menlow, and Randall) Swinger Girl is suddenly standing right next to them. There was not enough time for her to have climbed down from the roof.
Continuity mistake: Near the end, Tyler T. and Tyler A. get their Tyler crests taken away. This stays as the camera follows them, but when the shot changes, all 4 Tyler crests are gone off the Tylers' jackets.